Introduction |
## ANTENNA NOTES FOR A DUMMY## Restricted Space Antennasby Walt Fair, Jr., W5ALT## Horizontal DipolesNow that we've gotten some preliminaries out of the way, we can start to look at some actual antennas. It seems that most antenna texts start out with dipole antennas, so that must be a pretty good starting point.
L (m) = 142.5/F(MHz) L (ft) = 468/F(MHz) Here's a question: What is the length of a center fed resonant dipole for the 6 meter band (50.1 MHz)?
In free space, the impedance of a center-fed half wave resonant dipole is about 72 ohms. That is a near perfect match for 75 ohm coax and quite acceptable for 50 ohm coax, too. Unfortunately, the impedance in the real world depends on the height above ground and the ground quality.
Note that for diameters larger than about 1 in, there is no perceptable affect of wire diameter and the antenna performs as if it were made with no losses. The biggest difference between the zero loss and copper wire cases occurs when the wire diameter is 0.25 in or less. Although the resonant length is a little smaller with copper wire, the major effect is an increase in impedance. This increase is mainly due to the wire resistance which increases in proportion to 1/D because of the skin effect. The following figure shows the effect on the antenna gain. What can we learn from this simple exercise? First, as far as a 40m dipole is concerned, there's no reason to use a conductor larger than about 1 in, but that's still too large for most installations, space limited or not. If we take 72 ohms as the radiation resistance, then the difference in impedance represents losses in the wire. To stay above 95% efficiency, we want the impedance to be less than about 72/0.95 = 76 ohms. From the graphs, that happens whenever the wire diameter is larger than about 0.03 inches, which corresponds to roughly AWG #20 wire. As long as the wire is larger than that, the effect is not going to be noticeable. We can cross check the results from the impedance calculation with the antenna gain results. Note that 95% efficiency is equivalent to 10 log(0.95) = 0.22 dB drop in gain. Since the free space gain of a lossless dipole is about 2.14 dBi, we are at 95% efficiency when the gain drops to about 1.92 dBi. That happens with a wire diameter less than about 0.03 in, confirming the earlier evaluation. These results can be scaled for other frequencies. For 80m, the limit will be approximately double the wire diameter or on the order of #14 AWG wire.
First, notice that the resonant length and impedance both vary quite a bit depending on the ground conditions, which we normally have no control over. The resonant length can vary from 66 to 68 ft, with the larger variations over a perfectly conducting ground. The impedance also will vary from very low to around 100 ohms, with the perfect ground showing the larger variations. Therefore, in building a dipole, we shouldn't be too concerned about exactly estimating the resonant length or the impedance. We'll always have to adjust the length for resonance and the impedance will generally be in an acceptable range, unless we are over a perfect ground. But what about gain and perfomance? As the above figures show, there are also variations in the gain and the take off angle. First, notice that at heights below about 30 ft, the take off angle is 90 degress - straight up. That means that most of the radiation is going vertically upward, so the antenna will be less than optimum for DX contacts. If we want to obtain a low angle of radiation, say 20 to 30 degrees, then we better invest in some tall towers! Surprisingly, the poorer ground shows the lower takeoff angles at lower heights, so that may be good. Notice in the gain graph that the curves for less than ideal ground conditions are lower than that for a perfect ground. The model for a perfect ground has no ground losses - all energy is reflected. The difference between the curves shows the loss due to real world ground conditions. As can be seen, there is at least a 1 dB loss from perfect to average ground and another 1 dB loss from average to poor ground. Remember that each dB represents about 20% loss of the radiated power. But worse for limited space conditions is the situation at low heights. At heights less than about 30 ft, losses can be on the order of 4 to 6 dB. That amounts to losing about 60 - 75% of the radiated power warming the ground, while most of the rest of the power warms the clouds overhead! L = 492/F(MHz) = 468/50.1 = 9.34 ft or 9 ft 4 in, approximately Due to conductor size, etc., the actual length will vary |

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